Iran's Supreme Court has upheld a five-year jail sentence imposed on a British-Iranian charity worker for allegedly violating national security, her family said on April 24.

Nazanin Zaghari-Ratcliffe, a project manager with the Thomson Reuters Foundation, was arrested by the Islamic Revolutionary Guards Corps in April 2016 at Tehran's airport as she was about to return to Britain with her 2-year-old daughter after a family visit.

Iranian media have said she was convicted of plotting the "soft overthrow" of Iran's clerical establishment during widespread protests that followed the reelection of former hard-line president Mahmud Ahmadinejad in 2009.

The foundation and her family deny the charges. Her family said the Revolutionary Guards tried to get her to confess on camera that she had trained and recruited spies, something she refused to do.

After two secret trials, she was sentenced to five years' imprisonment in September.

Her husband, Richard Ratcliffe, said the appeal was her last legal opportunity to challenge the sentence.

"Nazanin discovered this weekend that her final appeal at the Supreme Court has failed and her five-year sentence has been upheld," he said, describing his wife as angry but not shocked.

"Her lawyer was told over the phone that there was no more that the court could do for Nazanin’s case and the legal review was closed. There was no court hearing for this judgment."

The Iranian judiciary declined to respond to calls seeking comment.

Ratcliffe said his wife still does not know the exact charges on which she was convicted.

"Nazanin is innocent. She was a mum on holiday, who works for a development charity in London," he said.

Britain's Foreign Office said it is "deeply concerned" by reports that the appeal was rejected.

"The prime minister and foreign secretary have both raised Mrs. Zaghari-Ratcliffe's case with their counterparts in Iran," a Foreign Office spokesperson said.

"We continue to press the Iranians for access and for due process to be followed and are ready to help get her daughter back safely to the U.K. if requested."

The Foreign Office has previously expressed "deep concern" over Zaghari-Ratcliffe's sentence but has stopped short of calling for her release.

The Thomson Reuters Foundation is a London-based charity that operates independently from Reuters News. Its chief executive, Monique Villa, described the rejection of the appeal as a big blow.

"She is not a spy but an innocent mother who traveled to Iran only to show her baby to her parents. I stand united with Richard in calling for her immediate release. Nazanin has suffered terribly over her past year," Villa said.

"We continue to be very concerned for her health and well-being, and she is desperately missed by her family and all at the foundation. I ask for clemency."

Iran refuses to recognize dual nationals and denies them access to consular assistance. The British ambassador to Iran last year visited Zaghari-Ratcliffe's daughter Gabriella, who has been placed in the care of her Iranian grandparents.

Last year, the United Nations human rights investigator for Iran called for the immediate release of three Iranians with dual nationality, including Zaghari-Ratcliffe.

"Iran has now detained Nazanin for more than a year on vague charges without basic respect for due process," said Sarah Leah Whitson, Middle East director at Human Rights Watch. "They should release her immediately and let her reunite with her family."

Zaghari-Ratcliffe is one of several dual nationals held in Iran by hard-liners in the country's judiciary and security services on espionage charges. Analysts say Tehran may hope to use the detainees as bargaining chips in future negotiations with the West.

Among the dual nationals being held in Iran are Iranian-American businessman Siamak Namazi and his father, Baquer Namazi, who are serving 10-year prison sentences for "cooperating with the hostile American government."

Iranian-American Robin Shahini is serving an 18-year prison sentence for "collaboration with a hostile government," though he recently received bail.

Yet to be tried is Iranian-American Karan Vafadari, an art gallery manager held along with his Iranian wife.

Iranian-Canadian national Abdolrasoul Dorri Esfahani, a member of the country's team that negotiated the nuclear deal, is believed to have been indicted.